1. Field of the Invention
The present invention relates to longitudinally coupled surface acoustic wave resonator filters and, more particularly, to a longitudinally coupled surface acoustic wave resonator filter in which the electrode structure is improved so that the selectivity is increased.
2. Description of the Related Art
Recently, surface acoustic wave (SAW) filters have been used as band-pass filters for various communication devices. Such band-pass filters require a suitable pass band width and high selectivity. In addition, SAW filters used in radio-frequency (RF) stages require that a pass band be within a desired range and that high selectivity be achieved.
In particular, in the case of SAW filters used for RF stages in cordless telephone sets in accordance with the CT-1, CT-1+ and CT-2 standards, it is important to achieve a sufficiently large attenuation at points .+-.20 MHz and .+-.40 MHz apart from a center frequency, although the desired attenuation varies depending upon intermediate frequency (IF) filters used in the telephone sets. Thus, filters having higher selectivity are required. Also with respect to IF filters, characteristics of selectivity between adjacent channels are important.
On the other hand, it is known that, among SAW filters, those using SAW resonator filters are particularly effective in reducing the insertion loss, in increasing the stop-band attenuation and in reducing the size. For example, a filter has been proposed which includes longitudinally coupled SAW resonator filters connected in a multi-stage arrangement. Each SAW resonator filter is formed by arranging three interdigital transducers (IDTs) or three sets of interdigital transducers at equal intervals in a surface wave propagation direction on a piezo-electric substrate formed of a 36.degree.-rotated Y-cut X-propagation LiTaO.sub.3 or the like and by providing reflector electrodes at the opposite sides of the IDTs arrangement region which extend in the surface wave propagation direction. The filter thus arranged can operate with a smaller insertion loss and can be smaller in size.
In the above-described conventional longitudinally coupled SAW resonator filter, however, the attenuation in a stop-band which is higher than the pass band cannot be sufficiently increased.
FIG. 7 shows an example of an insertion loss-frequency characteristic of a filter including the above-described conventional longitudinally coupled SAW resonator filters connected in a two-stage arrangement. In FIG. 7, an essential portion of the characteristic represented by the solid line A is replotted as represented by the solid line B by enlarging the scale of the insertion loss, that is, using the scale on the right-hand side of the ordinate.
The characteristic curve shown in FIG. 7, representing the characteristic of the structure formed of the conventional longitudinally coupled SAW resonator filters in a two-stage connection having a pass band set at 864 to 868 MHz, has an increase at a frequency higher than the pass band due to a response of the IDTs (as indicated by arrow C in FIG. 7), which shows that the attenuation in the range of about 886 to 906 MHz is not sufficiently large.
The above-mentioned response of the IDTs is caused by internal reflection in the IDTs. Therefore, the level of the response depends upon the film thickness and line width of each of the fingers of the IDTs. For this reason, the method of increasing the film thickness of electrode fingers or reducing the line width of electrode fingers in a manufacturing process to set the desired attenuation in the above-mentioned range has been used as a method for reducing the level of undesirable response of the IDTs.
However, if the film thickness of the electrode fingers is increased, the propagation loss of surface waves propagating in the IDTs is increased, resulting in an increase in insertion loss. If the line width of the electrode fingers is increased, the electrical resistance through each electrode finger is increased, also resulting in an increase in insertion loss.
As conventional design methods for increasing the selectivity of SAW filters, a new phase weighting method (Shingaku Giho US81-22) and a method of forming a split electrode structure in some of the IDTs to limit internal reflection in the IDTs are known.
Of these methods, however, the former method requires a troublesome operation for designing weighted IDTs and also requires that an increased number of electrode fingers be thinned out. Therefore, this method inherently experiences the problem of an increase in impedance or an increase in the length of the IDTs.
The latter method uses a split electrode having electrode fingers, the width of which is half the line width of the ordinary electrode fingers, i.e., single electrodes. Therefore, this method requires advanced processing techniques such as dry etching techniques if it is used for filters of 800 MHz or higher bands. It also experiences the problem of a reduction in yield mainly because it is difficult to control the line width of electrode fingers of each IDT in the mixed arrangement of the single electrodes and the split electrodes.